The present invention relates to a can body forming process and equipment therefor in which the opposite ends of a can body blank of a metal sheet are joined together with thermoplastic organic polymer adhesive to be formed into a can body and, more particularly, to such a process and equipment which can produce a sound can body joint substantially free from internal strain and which can maintain a high forming accuracy without requiring any particular adjustment so as to always ensure forming of correct can bodies.
Recently, as containers of carbonated beverages and fruit juice beverages, so-called lapped-seam cans made of tin-free steel and having their body seam joined in a lapped state with adhesive of thermoplastic organic polymer have come to be used in large quantities.
As compared with soldered cans which have been used from very early days, the lapped-seam cans using adhesive are hygienically advantageous because fine particulate solder does not deposit on the inside of cans. Further, in the lapped-seam cans, cost can be reduced remarkably because tin-free steel is used, and printing can be made almost at any parts of their outside surface because they do not require a varnish-free zone required in the case of soldered cans. However, it is required for the lapped-seam cans that the conditions under which the adhesive is heated, pressure-bonded and cooled should be determined and controlled more severely as compared with the former.
That is to say, in most cases, polyamide adhesive films are used as the thermoplastic organic polymer adhesive, and such adhesive films can produce sound adhesion if suitable conditions are selected. However, if fine water drops are deposited on the adhesive film, bubbles may possibly be formed in the adhesive layer due to evaporation of such water drops occurrable through the steps of heating, melting and contact-bonding of the adhesive.
If such defects as bubbles exist in the adhesive layer, for example, at upper and lower ends of the can body, since the adhesive layer is broken by the can body flanging and succeeding can end seaming steps, minute through-holes will be formed in the adhesive layer, thus causing bacterial contamination of the can content or so-called micro-leak, namely, exudation of the contents. Also, if the bubbles exist in the adhesive layer in its middle portion of the can body, the side seam of the can body may be separated apart and the contents may be leaked out when the internal pressure of the can is increased with the filled carbonated beverage.
As a method proposed so far for manufacturing the aforementioned lapped-seam cans, a can body blank having its opposite ends coated with polyamide adhesive films is first brought into contact with a can body forming cylinder which is under forcibly-cooled conditions over the entire length thereof. Then, said opposite ends as they are cooled from inside are with high frequency induction heating heated and lapped one on the other, and the thus overlapped portions are hammered with a forcibly liquid-cooled hammer to quench and cure the adhesive. However, in the aforementioned method according to the prior art, since the can forming cylinder is forcibly cooled over the entire length thereof, the can body blank transferred in close contact with the cylinder is always in a cooled state. Therefore, to raise the temperature of the polyamide adhesive coated on the opposite ends of the blank to a temperature higher than 230.degree. C. required for melting the adhesive, the heating time must be longer than that of a blank at normal temperatures. If the high frequency induction heating voltage is increased to shorten the heating temperature, the adhesive can be heated in a very short time, but a great undulated deformation occurs at end portions of the blank and even contact-bonding becomes impossible due to a large temperature difference existing between the end and middle portions. Further, the aforementioned prior art method has the following shortcomings.
That is to say, the forced-cooling causes frost on the surface of the can body forming cylinder.
If such frost deposits on the surface of the can body forming cylinder, since frost also deposits on the can body blank when being transferred along the forming cylinder and water drops are condensed in the adhesive, the water drops cause foam therein and reduce the adhesive strength and, thus, it is impossible to maintain the accuracy of the cylinder diameter and, consequently, the diameter of the can body blank transferred in a state wound around the cylinder becomes unstable. Therefore, an excessive force will be exerted thereto in the flanging and can end seaming steps succeeding to the can body forming process and the lapped joint may be accidentally broken or the rolled portions may be creviced, causing accidental leakage.